IEC TS 62788-2:2024

Measurement procedures for materials used in photovoltaic modules - Part 2: Polymeric materials - Frontsheets and backsheets

IEC TS 62788-2:2024 defines test methods and datasheet reporting requirements for safety and performance-related properties (mechanical, electrical, thermal, optical, chemical) of non‑rigid polymeric materials intended for use in terrestrial photovoltaic modules as polymeric front- and backsheets. The test methods in this document define how to characterize front- and backsheet materials and their components in a manner representative of how they will be used in the module, which eventually includes combination with other matched components such as encapsulants or adhesives. Results of testing described in this document are called by IEC 62788-2-1 for safety qualification of polymeric front- and backsheets on component level and support the safety and performance-related tests defined on the PV module level as defined in the series IEC 61730 (for safety) and IEC 61215 (for performance). This document also defines test methods for assessing inherent material characteristics of polymeric front- and backsheets or their components, which can be required in datasheet reporting or can be useful in the context of product development or design of PV modules.
This second edition cancels and replaces the first edition published in 2017. This edition includes the following significant technical changes with respect to the previous edition:
a) With revision of IEC 61730-1 the requirements for the polymeric front- and backsheet have been moved from IEC 61730-1 into IEC 62788-2-1. This is reflected accordingly.
b) The tensile testing method has been refined based on findings of round robin tests, including updated drawings.
c) A thermal pre-exposure method has been introduced to be equivalent to the thermal effects of a "lamination" cycle. This pre-exposure defines the "fresh" state of the front- or backsheet in final application for evaluation of changes in ageing tests. For practical reasons, an oven exposure has been defined as an equivalent test.
d) The multiple functions of the lamination protrusion test (previously DTI test) have been clarified, to identify and measure RUI layer thickness as well as to identify layers for which the comparative tracking index (CTI) needs to be determined. Also the content of IEC 62788-2-1 has been updated, by which the lamination protrusion test and MST 04 are additionally set in perspective to each other via engineering judgement.
e) The DC breakdown voltage test method has been updated and the option to perform a withstand voltage test has been added (to reduce the required measurement voltage). The correction of DC breakdown voltage ( ) measurements, needed in the presence of non‑RUI layers and after the lamination protrusion test, has been defined more precisely.
f) Details for thickness measurement have been added (engineered surface roughness due to embossing).
g) The adhesion test methods have been reviewed and updated. The single cantilevered beam test has been added. Figures have been updated to align with IEC 62788-1-1.
h) The thermal failsafe test has been added as a test method based on discussion in the parallel project for IEC 62788-2-1. The test method offers a single temperature-point evaluation to include elongation at break to the thermal endurance evaluation.
I) A sequential UV/TC test ("solder bump test") has been added.

General Information

• Status: Published
• Publication Date: 11-Jul-2024
• ICS: 27.160 - Solar energy engineering
• Technical Committee: TC 82 - Solar photovoltaic energy systems
• Drafting Committee: WG 2 - TC 82/WG 2
• Current Stage: PPUB - Publication issued
• Start Date: 12-Jul-2024
• Completion Date: 25-Aug-2023

Relations

• Revises: IEC TS 62788-2:2017 - Measurement procedures for materials used in photovoltaic modules - Part 2: Polymeric materials - Frontsheets and backsheets
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC TS 62788-2 ®
Edition 2.0 2024-07
TECHNICAL
SPECIFICATION
Measurement procedures for materials used in photovoltaic modules –
Part 2: Polymeric materials – Frontsheets and backsheets

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IEC TS 62788-2 ®
Edition 2.0 2024-07
TECHNICAL
SPECIFICATION
Measurement procedures for materials used in photovoltaic modules –

Part 2: Polymeric materials – Frontsheets and backsheets

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160  ISBN 978-2-8322-8872-6

– 2 – IEC TS 62788-2:2024 © IEC 2024
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 8
3 Terms and definitions . 10
4 Test procedures . 13
4.1 General . 13
4.1.1 Purpose . 13
4.1.2 Sample pre-treatment . 14
4.1.3 Type of sample constructions . 15
4.2 Mechanical characteristics . 16
4.2.1 General . 16
4.2.2 Thickness . 16
4.2.3 Area weight . 17
4.2.4 Tensile properties . 17
4.3 Adhesion testing . 21
4.3.1 Purpose . 21
4.3.2 General . 21
4.3.3 Specific use cases . 22
4.3.4 Methods . 24
4.3.5 Reporting requirements . 29
4.4 Thermal characteristics . 30
4.4.1 Thermal endurance . 30
4.4.2 Thermal failsafe test . 31
4.4.3 Dimensional stability . 32
4.4.4 Relative thermal expansion . 34
4.4.5 Thermal conductivity . 34
4.5 Electrical characteristics and insulation thickness . 35
4.5.1 Breakdown voltage . 35
4.5.2 Lamination protrusion test (aka DTI test) . 40
4.5.3 Comparative tracking index (CTI) . 43
4.5.4 Volume resistivity . 44
4.6 Optical characteristics . 45
4.6.1 General . 45
4.6.2 Specimen preparation . 46
4.6.3 Visual inspection . 46
4.6.4 Optical transmittance . 47
4.6.5 Optical reflectance . 48
4.6.6 Yellowness index . 48
4.6.7 Colour measurement . 49
4.6.8 Surface gloss. 50
4.7 Diffusion characteristics . 51
4.7.1 Permeability of water vapour . 51
4.7.2 Permeability of oxygen . 52
4.8 Chemical characteristics . 52
4.8.1 Resistance to solvents . 52
4.9 Other characteristics . 54
4.9.1 Ignitability – Purpose . 54

4.9.2 Flammability – Purpose . 54
4.10 Accelerated ageing tests . 54
4.10.1 Purpose . 54
4.10.2 Damp-heat ageing test . 55
4.10.3 UV weathering . 56
4.10.4 Abrasion test . 61
4.11 Sequential UV/TC stress test ("solder bump test") . 61
4.11.1 Purpose . 61
4.11.2 Sample construction . 62
5 Uniform characterization form . 68
5.1 General . 68
5.2 Layer stack description . 68
5.3 Material test results and reporting requirements. 68
6 Data sheet . 70
6.1 Purpose . 70
6.2 Reporting requirements . 70
7 Product identification sheet (label) . 70
Annex A (normative) Sample preparation . 71
A.1 Purpose . 71
A.2 Sample constructions . 71
A.2.1 General considerations . 71
A.2.2 Materials and procedures . 71
A.2.3 Overview of sample pre-treatments . 77
Annex B (informative) Overview of adhesion testing of front- and backsheets . 79
B.1 General . 79
B.2 Type of adhesion failure . 79
B.3 Characteristics of peel tests . 80
B.4 Characteristics of pluck and lap-shear tests . 80
B.5 Characteristics of cross-hatch tape test . 81
B.6 Characteristics of single cantilevered beam (SCB) test . 81
B.7 Other caveats . 81
Annex C (informative) Abrasion testing . 84
C.1 Purpose . 84
C.2 Methods . 84
Annex D (informative) Visual examination after sequential UV/TC stress test . 85
D.1 General . 85
D.2 Airside inspection of backsheet in reflection mode . 85
D.2.1 General . 85
D.2.2 Unexposed samples . 85
D.2.3 Airside: Rating of 1 . 86
D.2.4 Airside: Rating of 2 . 87
D.2.5 Airside: Rating of 3 . 87
D.2.6 Airside: Rating of 4 . 88
D.3 Glass side inspection of backsheet in transmission mode . 89
D.3.1 Unexposed samples . 89
D.3.2 Glass-side: Rating of A . 89
D.3.3 Glass-side: Rating of B . 90
D.3.4 Glass-side: Rating of C . 91

– 4 – IEC TS 62788-2:2024 © IEC 2024
D.3.5 Glass-side: Rating of D . 92
Bibliography . 93

Figure 1 – Specimen dimensions and markings in tensile test (unstrained and strained) . 20
Figure 2 – Fixation of the sample for 180° peel (for use with flexible/flexible samples) . 25
Figure 3 – Single and multiple failure modes in a peel test . 26
Figure 4 – Sheet sandwich (top) for preparation of T-peel test specimens (bottom) . 27
Figure 5 – Illustration of area removal by tape in cross-hatch test, with classification
from 0 to 5 (from left to right) . 28
Figure 6 – Specimen before and after exposure . 33
Figure 7 – Equal electrodes for dielectric strength test . 36
Figure 8 – Schematics of test specimen for DTI test before and after lamination . 41
Figure 9 – Example of DTI cross-section of a backsheet . 42
Figure 10 – Schematic indicating need for CTI measurement of materials A and B to
determine creepage distances . 43
Figure 11 – Side-view schematic of solder-bump coupon sample after lamination . 62
Figure 12 – Example of "solder-bump coupons" in a specimen holder. . 64
Figure B.1 – Adhesive and cohesive failure type schematics for lap shear (top) and T-
pluck (bottom) . 79
Figure D.1 – Example of an open area with scratches. . 86
Figure D.2 – Example of grey residue from the solder wire in the trench. . 86
Figure D.3 – Example contamination (at different magnifications) . 87
Figure D.4 – Airside microscope image of ridge area with superficial "stretch marks" . 87
Figure D.5 – Airside microscope images with cracks parallel to the ridge region . 88
Figure D.6 – Airside microscope images with cracks on the ridge and in the trench . 88
Figure D.7 – Airside image of a crack through all the RUI layers . 89
Figure D.8 – Observation of delamination artefact at the ends of the trench . 89
Figure D.9 – Backlit microscope image of a mark on the ridge of a transparent
backsheet suspected to be a scratch as opposed to a degradation related crack . 90
Figure D.10 – Examples of cracks smaller than 2 mm at the perimeter . 91
Figure D.11 – Cracks greater than 2 mm at the perimeter and near ridge and trench . 92
Figure D.12 – Cracks across the open space . 92

Table 1 – Overview of typical front- and backsheet building blocks . 15
Table 2 – Overview of adhesion tests . 22
Table 3 – Overview of sample preparation for exposure of sun-facing (inner) side of
backsheet depending on intended post evaluation (see Table A.1) . 58
Table 4 – UV exposure conditions . 59
Table 5 – Characterization categories . 66
Table 6 – Example table for recording inspection results after all intermediate UV and
TC cycles with three test cycles of the sequential UV/TC stress test . 67
Table 7 – Uniform characterization form (UCF) for polymeric PV front- or backsheet. . 69
Table A.1 – Overview of sample preparation suitable for material tests . 73
Table A.2 – Sample pre-treatments . 77
Table B.1 – Overview of various adhesion tests and their general capability of testing
polymeric front- and backsheet . 82

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MEASUREMENT PROCEDURES FOR MATERIALS
USED IN PHOTOVOLTAIC MODULES –

Part 2: Polymeric materials – Frontsheets and backsheets

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC TS 62788-2 has been prepared by IEC Technical Committee 82: Solar photovoltaic energy
systems. It is a Technical Specification.
This second edition cancels and replaces the first edition published in 2017. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) With revision of IEC 61730-1 the requirements for the polymeric front- and backsheet have
been moved from IEC 61730-1 into IEC 62788-2-1. This is reflected accordingly.
b) The tensile testing method has been refined based on findings of round robin tests, including
updated drawings.
c) A thermal pre-exposure method has been introduced to be equivalent to the thermal effects
of a "lamination" cycle. This pre-exposure defines the "fresh" state of the front- or backsheet
in final application for evaluation of changes in ageing tests. For practical reasons, an oven
exposure has been defined as an equivalent test.

– 6 – IEC TS 62788-2:2024 © IEC 2024
d) The multiple functions of the lamination protrusion test (previously DTI test) have been
clarified, to identify and measure RUI layer thickness as well as to identify layers for which
the comparative tracking index (CTI) needs to be determined. Also the content of
IEC 62788-2-1 has been updated, by which the lamination protrusion test and MST 04 are
additionally set in perspective to each other via engineering judgement.
e) The DC breakdown voltage test method has been updated and the option to perform a
withstand voltage test has been added (to reduce the required measurement voltage). The
correction of DC breakdown voltage ( V ) measurements, needed in the presence of
BD
non-RUI layers and after the lamination protrusion test, has been defined more precisely.
f) Details for thickness measurement have been added (engineered surface roughness due to
embossing).
g) The adhesion test methods have been reviewed and updated. The single cantilevered beam
test has been added. Figures have been updated to align with IEC 62788-1-1.
h) The thermal failsafe test has been added as a test method based on discussion in the
parallel project for IEC 62788-2-1. The test method offers a single temperature-point
evaluation to include elongation at break to the thermal endurance evaluation.
i) A sequential UV/TC test ("solder bump test") has been added.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
82/2109/DTS 82/2169/RVDTS
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Specification is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 62788 series, published under the general title Measurement
procedures for materials used in photovoltaic modules, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

MEASUREMENT PROCEDURES FOR MATERIALS
USED IN PHOTOVOLTAIC MODULES –

Part 2: Polymeric materials – Frontsheets and backsheets

1 Scope
This part of IEC 62788 defines test methods and datasheet reporting requirements for safety
and performance-related properties (mechanical, electrical, thermal, optical, chemical) of
non-rigid polymeric materials intended for use in terrestrial photovoltaic modules as polymeric
front- and backsheets.
The test methods in this document define how to characterize front- and backsheet materials
and their components in a manner representative of how they will be used in the module, which
eventually includes combination with other matched components such as encapsulants or
adhesives. It is impractical to conduct all characterization and endurance tests for the front- or
backsheet component on the module level. Instead, testing is performed directly on these
components or on dedicated test coupons prepared under comparable processing conditions
(i.e. lamination) as for PV modules.
Results of testing described in this document are called by IEC 62788-2-1 for safety
qualification of polymeric front- and backsheets on component level and support the safety and
performance-related tests defined on the PV module level as defined in the series IEC 61730
(for safety) and IEC 61215 (for performance). This document also defines test methods for
assessing inherent material characteristics of polymeric front- and backsheets or their
components, which can be required in datasheet reporting or can be useful in the context of
product development or design of PV modules.
Backsheets provide the electrical insulation at the backside of a photovoltaic (PV) module under
the environmental stress factors and use conditions encountered during the intended lifetime
of the module. Frontsheets have the same function at the sun-facing side of the module. Both
can be made from glass or polymeric material.
Polymeric front- and backsheets are typically compositions of layered materials, such as films,
adhesives or coatings, in which at least one material layer delivers the relied-upon insulation
(RUI) for electrical safety. Other layers can provide extended protection of the RUI against the
environmental factors or adhesive functionality. As an integral part of the PV module, the front-
or backsheet provides their durable electrical insulating function in the presence of the other
components of the PV module, such as solar cells, electrical circuits and connectors,
encapsulant, sealing material, and junction boxes. These elements can introduce additional
stresses on the front- or backsheet (e.g. by chemical interactions or introducing thermal load or
mechanical stress) or alter environmental stresses (e.g. filtering of ultraviolet radiation reaching
the sun-facing side of the backsheet).
Material characterization in this document is performed on unaged samples (after a thermal
pre-exposure) or after accelerated ageing including thermo-oxidative ageing (thermal
endurance and failsafe tests), hydrolytic ageing (damp heat), photolytic ageing (UV weathering),
abrasion, and sequential UV ageing plus thermal cycling.
Rigid polymeric sheet materials (also providing mechanical support) can require further
consideration, which is outside the scope of this document.

– 8 – IEC TS 62788-2:2024 © IEC 2024
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60050-581, International Electrotechnical Vocabulary – Part 581: Electromechanical
components for electronic equipment
IEC 60050-826, International Electrotechnical Vocabulary – Part 826: Electrical installations
IEC 60112, Method for the determination of the proof and the comparative tracking indices of
solid insulating materials
IEC 60212, Standard conditions for use prior to and during the testing of solid electrical
insulating materials
IEC 60216-1, Electrical insulating materials – Thermal endurance properties – Part 1: Ageing
procedures and evaluation of test results
IEC 60216-2, Electrical insulating materials – Thermal endurance properties – Part 2:
Determination of thermal endurance properties of electrical insulating materials – Choice of test
criteria
IEC 60216-5, Electrical insulating materials – Thermal endurance properties – Part 5:
Determination of relative thermal endurance index (RTE) of an insulating material
IEC 60243-1, Electrical strength of insulating materials – Test methods – Part 1: Tests at power
frequencies
IEC 60243-2, Electric strength of insulating materials – Test methods – Part 2: Additional
requirements for tests using direct voltage
IEC 60296, Fluids for electrotechnical applications – Unused mineral insulating oils for
transformers and switchgear
IEC 60664-1, Insulation co-ordination for equipment within low-voltage systems – Part 1:
Principles, requirements and tests
IEC 60904-3, Photovoltaic devices – Part 3: Measurement principles for terrestrial photovoltaic
(PV) solar devices with reference spectral irradiance data
IEC 61140, Protection against electric shock – Common aspects for installation and equipment
IEC 61215-2, Terrestrial photovoltaic (PV) modules – Design qualification and type approval –
Part 2: Test procedures
IEC 61730-1, Photovoltaic (PV) module safety qualification – Part 1: Requirements for
construction
IEC TS 61836, Solar photovoltaic energy systems – Terms, definitions and symbols
IEC 62788-1-4, Measurement procedures for materials used in photovoltaic modules –
Part 1-4: Encapsulants – Measurement of optical transmittance and calculation of the solar-
weighted photon transmittance, yellowness index, and UV cut-off wavelength

IEC 62788-1-5, Measurement procedures for materials used in photovoltaic modules –
Part 1-5: Encapsulants – Measurement of change in linear dimensions of sheet encapsulation
material resulting from applied thermal conditions
IEC 62788-2-1, Measurement procedures for materials used in photovoltaic modules –
Part 2-1 Polymeric materials – frontsheet and backsheet – Safety requirements
IEC 62788-6-2, Measurement procedures for materials used in photovoltaic modules – Part 6-2:
General tests – Moisture permeation testing of polymeric materials
IEC TS 62788-6-3, Measurement procedures for materials used in photovoltaic modules –
Part 6-3: Adhesion Testing of Interfaces within PV Modules
IEC TS 62788-7-2, Measurement procedures for materials used in photovoltaic modules –
Part 7-2: Environmental exposures – Accelerated weathering tests of polymeric materials
IEC 62790, Junction boxes for photovoltaic modules – Safety requirements and tests
IEC 62805-2, Method for measuring photovoltaic (PV) glass – Part 2: Measurement of
transmittance and reflectance
ISO 291, Plastics – Standard atmospheres for conditioning and testing
ISO 527-1, Plastics – Determination of tensile properties – Part 1: General principles
ISO 527-3, Plastics – Determination of tensile properties – Part 3: Test conditions for films and
sheets
ISO 536, Paper and board – Determination of grammage
ISO 1519, Paints and varnishes – Bend test (cylindrical mandrel)
ISO 1520, Paints and varnishes – Cupping test
ISO 2409, Paints and varnishes – Cross-cut test
ISO 2813, Paints and varnishes – Determination of gloss value at 20 degrees, 60 degrees and
85 degrees
ISO 4593, Plastics – Film and sheeting – Determination of thickness by mechanical scanning
ISO 11359-1, Plastics – Thermomechanical analysis (TMA) – Part 1: General principles
ISO 11359-2, Plastics – Thermomechanical analysis (TMA) – Part 2: Determination of
coefficient of linear thermal expansion and glass transition temperature
ISO 11664-1, Colorimetry – Part 1: CIE standard colorimetric observers
ISO 11664-2, Colorimetry – Part 2: CIE standard illuminants
ISO 11664-4, Colorimetry – Part 4: CIE 1976 L*a*b* Colour space
ISO 15105-2, Plastics – Film and sheeting – Determination of gas-transmission rate – Part 2:
Equal-pressure method
– 10 – IEC TS 62788-2:2024 © IEC 2024
ISO 15106-1, Plastics – Film and sheeting – Determination of water vapour transmission rate –
Part 1: Humidity detection sensor method
ISO 15106-2, Plastics – Film and sheeting – Determination of water vapour transmission rate –
Part 2: Infrared detection sensor method
ISO 15106-3, Plastics – Film and sheeting – Determination of water vapour transmission rate –
Part 3: Electrolytic detection sensor method
ISO 17223, Plastics – Determination of yellowness index and change in yellowness index
ISO 22007-4, Plastics – Determination of thermal conductivity and thermal diffusivity – Part 4:
Laser flash method
ASTM D374, Standard Test Methods for Thickness of Solid Electrical Insulation
ASTM D7869, Standard Practice for Xenon Arc Exposure Test with Enhanced Light and Water
Exposure for Transportation Coatings
UL 746B, Standard for Polymeric Materials – Long Term Property Evaluations
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-581,
IEC 60050-826, IEC 60664-1, IEC 61140, IEC 61730-1, and IEC TS 61836, together with the
following, apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
abrasion resistance
ability of a material to withstand mechanical action such as rubbing, scraping, or erosion, which
tends to progressively remove material from its surface
Note 1 to entry: See IEC 62788-7-3 for abrasion testing methods.
3.2
adhesive failure
de-bonding occurring between the adhesive and the adherent, to be differentiated from
cohesive failure within the adhesive material
Note 1 to entry: See also Annex B.
3.3
air-side
side of the front- or backsheet oriented towards the outside of the PV module, i.e., away from
the cells
3.4
backsheet
BS
(combination of) outer layer(s) of the PV module, located as substrate on the back of the PV
module (designed for prolonged use only with indirect or limited direct sunlight < 300 W/m )
and providing protection of the inner components of the PV module from external stresses and
weather elements, as well as providing electrical insulation
3.5
breakdown voltage
V
BD
DC voltage at which electric breakdown occurs under prescribed test conditions, or in use
Note 1 to entry: Breakdown voltage testing in the context of PV modules and component materials applies direct
current (DC).
[SOURCE: IEC 60050-212:2010, 212-11-34, modified – added symbol, added "DC" in the
definition, and added Note 1 to entry.]
3.6
cohesive failure
crack propagating within the adhesive during adhesion test, e.g., peel test
3.7
comparative tracking index
CTI
numerical index value related to the maximum voltage that a material can withstand without
formation of a permanent and electrically conductive carbon (tracking) path and without a
persistent flame occurring, when evaluated under specified test conditions defined in IEC 60112
Note 1 to entry: The mentioned maximum test voltage is not in conjunction with any system or operational voltage,
but it is used for evaluation of material groups.
[SOURCE: IEC 60050-212:2010, 212-11-59, modified – The definition has been rephrased by
also clarifying that CTI is an index value to evaluate material groups according to IEC 60112.
Note 1 to entry has also been added.]
3.8
directions of films, specimens and cracks
3.8.1
machine direction
MD
direction along which the material layer was extruded or produced, extending out of a die or
other manufacturing equipment in a production line
3.8.2
transverse direction
TD
direction perpendicular to which the material layer was extruded or produced
3.9
distance through insulation
t
DTI
thickness of relied-upon insulation (RUI) after the lamination protrusion test, with the minimum
allowable value defined by the maximum working voltage

– 12 – IEC TS 62788-2:2024 © IEC 2024
3.10
elongation at break
ɛ
B
strain at which the specimen under test breaks
3.11
frontsheet
FS
(combination of) outer layer(s) of the PV module designed for prolonged exposure to direct
sunlight (> 300 W/m ) and providing protection of the inner components of the module from
external stresses and weather elements, as well as providing electrical insulation
3.12
inner side
side of the front- or backsheet that is oriented to the solar cells, typically laminated to the
encapsulant
3.13
material group
category of insulation materials according to IEC 60664-1 as defined by the results of the CTI
test
3.14
polymeric material
materials that are either natural or synthetic and are primarily composed of chained molecules
of monomers, combinations of monomers, and combined polymers and may contain cross-
linking agents, fillers, colorants, and other materials
3.15
release material
rm
film material that is inserted in a layer stack before lamination to render inactive the adhesion
between interfaces
Note 1 to entry: Examples of suitable release materials are fluoropolymer sheets (e.g. PTFE, FEP, ETFE) as well
as silicon treated sheets.
3.16
relied-upon insulation
RUI
solid insulation system providing protection against electric shock in the final application, with
the material's requirements for thermal endurance and resistance against environmental stress
factors
Note 1 to entry: Thin-films used as polymeric front- or backsheet can consist of RUI plus additional layers that have
other functions, e.g., they protect the polymeric materials from UV radiation.
3.17
relative thermal endurance index
RTE
numerical value of the Celsius temperature expressed in degrees Celsius at which the
estimated time to endpoint of an insulating material is the same as the estimated time to
endpoint of a control material at a temperature equal to its assessed thermal endurance
[SOURCE: IEC 60050-212:2010, 212-12-14, modified – Abbreviated term added and notes to
entry omitted.]
3.18
relative temperature index
RTI
temperature index of an insulating material or system obtained from the time which corresponds
to the known temperature index of a reference material or system when both are subjected to
the same ageing and diagnostic procedures in a comparative test
[SOURCE: IEC 60050-212:2010, 212-12-12]
3.19
sun-facing side
side of the front- or backsheet that is oriented in direction of the sun-facing front side of the PV
module
3.20
temperature index
TI
numerical value of the Celsius temperature expressed in degrees Celsius characterizing the
thermal capability of an insulating material or an insulation system
[SOURCE: IEC 60050-212:2010, 212-12-11, modified – Notes removed.]
3.21
tensile strength at break
σ
B
maximum engineering stress measured when a specimen is elongated in tension to the point of
breaking
3.22
transparent release material
TRM
release material with an AM1.5 photon weighted total transmission value of ≥ 85 % in the range
280 nm to 2 500 nm as well as in the range 300 nm to 400 nm
Note 1 to entry: Example of a suitable UV transparent release material is ETFE (ethylene tetra-fluoroethylene) and
perfluorinated ethylene propylene copolymer (FEP) film, both as pure formulations without UV absorbers and 50 μm
to 125 μm in thickness.
3.23
water vapour transmission rate
WVTR
rate of water vapour transport through the material per unit area induced by a unit vapour
pressure difference under specified temperature and humidity conditions
4 Test procedures
4.1 General
4.1.1 Purpose
Test procedures in this document are designed to evaluate properties of front- or backsheets,
or individual (sub)layers relied upon for insulation. Adhesion tests are provided to probe the
adhesion strength between various front- or backsheet layers. Procedures for environmental
stress exposures are included, with a schedule for post-stress evaluation testing.

– 14 – IEC TS 62788-2:2024 © IEC 2024
Purposes for which the data are intended to be used include:
• Evaluation of requirements for front- or backsheets as described in IEC 62788-2-1;
• Supplier driven comparison of front- or backsheet properties;
• Quality control testing of incoming backsheets used in PV module production;
• Guide to front- or backsheet product development.
The user is guided to be mindful of the purpose of testing, as not all tests will be useful for all
purposes. Recommendations for a data sheet are provided in the uniform characterization form
(UCF) provided in Clause 5.
This document stipulates ageing tests on engineering samples containing front- or backsheet
materials for the purpose of component endurance characterizati
...